The indictment of six men who worked for the Big Dig's largest concrete supplier is just a small step in the pursuit of contractors suspected of fraud on the multibillion-dollar highway project, authorities say.

The six current and former employees of Aggregate Industries were arrested Thursday on federal charges alleging they covered up the inferior quality of concrete delivered to the $14.6 billion project.

"This conduct is an affront to hard working Americans," U.S. Attorney Michael Sullivan said. "I think we've just scratched the surface in our effort to investigate the Big Dig."

The indictment charges the men with recycling concrete that was too old or already rejected by inspectors, and in some cases double-billing for the loads. The company was paid $105 million for 135,000 truckloads of concrete, and at least 5,000 truckloads did not meet specifications, according to the indictment.

Aggregate Industries said in a statement Thursday that it will cooperate with authorities.

"As a result of extensive testing by industry experts, Aggregate Industries is satisfied that all of the concrete it has supplied on the Big Dig and throughout the commonwealth is structurally sound," the statement said.

Sullivan said the safety of the project is not threatened and long-term maintenance problems are the most likely effect stemming from the substandard concrete.

The inferior concrete was used in spots throughout the project, including parts of the Interstate 93 and Interstate 90 tunnels, the indictment alleges.

The Big Dig, formally called the Central Artery and Third Harbor Tunnel project, buried Interstate 93 beneath downtown and connected the Massachusetts Turnpike to Logan Airport with a third tunnel beneath Boston Harbor. The project was plagued by long delays and cost overruns that ballooned from $2.6 billion to $14.6 billion. Earlier this year, after more than a decade of traffic detours, the last major section opened.

Stephen Delinsky, an attorney for one of the defendants, Gerard McNally, said prosecutors are looking for someone to blame for the troubled project.

"It's always easy to blame the lowest level, which is the concrete manufacturers," Delinsky said. "Each defendant believed that they acted in good faith. They believed at all times their conduct was legal and believed the concrete delivered to the Big Dig was appropriate."

Indicted were: former general manager Robert Prosperi, 63, of Lynnfield; Marc Blais, 36, of Lynn, a dispatch manager; John Farrar, 42, of Canterbury, Conn., a dispatch manager; McNally, 53, of Rockland, a quality control manager; Gregory Stevenson, 53, of Furlong, Pa., district operations manager; and Keith Thomas, 50, of Billerica, a dispatch manager.

Stevenson and Farrar are no longer with the company. Aggregate says it has suspended the other four.

The men were released on $100,000 unsecured bond each after making initial appearances in U.S. District Court in Boston.

Part of the scheme's undoing, according to Sullivan, was the company's own record-keeping. One shift of workers kept logs of the loads of the substandard concrete.

In some cases, concrete unused by other customers was delivered. The contractors were also required to pour the concrete within 90 minutes of mixing to prevent hardening. The logs showed the workers delivered concrete that exceeded that limit, sometimes with water added to make it look fresher, the indictment alleges.

Attorneys for the workers claim Big Dig managers sometimes waived the 90-minute rule when concrete trucks were lined up to meet a crushing demand for concrete. A spokesman for project manager Bechtel/Parsons Brinckerhoff declined comment.